What is the main mechanism of the dark current in terms of ion movement?

The dark current is a vital physiological mechanism in the phototransduction pathway, particularly in the context of rod photoreceptors. In darkness, photoreceptor cells maintain a depolarized state, which is achieved through a specific movement of ions.

The primary mechanism involves sodium ions (Na+) entering the outer segment of the photoreceptors. This influx of Na+ occurs through specialized ion channels known as cyclic nucleotide-gated (CNG) channels. In the dark, cyclic GMP (cGMP) levels are high, which keeps these channels open, allowing Na+ and some Ca2+ ions to flow into the photoreceptor cells.

As a result of this ion flow, the cell remains depolarized, generating a continuous release of neurotransmitters at the synaptic terminal. This view of the dark current emphasizes the crucial role of Na+ ions entering the outer segment, highlighting how the photoreceptors convert light stimuli into electrical signals by altering this ion movement when exposed to light.

Therefore, the option that identifies the movement of Na+ ions into the inner segment directly relates to the critical process sustaining the dark current, making it the correct choice.